For controlling the windscreen wipers in dependence on the wetting condition of a motor vehicle, windscreen sensors of various configurations are known.
In some cases detection of the wetting condition requires that the sensor gets in direct contact with the humidity, e.g. in the case of resistive or thermal sensors. Sensors which do not get in direct contact with humidity are in most cases mounted directly at the inner windscreen surface and utilize the change in the refraction index at the wetted windscreen surface (FIG. 1).
A light beam 6, in particular in the infrared range, is coupled by a sensor element 4 at a certain angle into the vehicle windscreen 1 such that the light beam 6 is reflected as completely as possible at the surface and can be received by a photo diode 4. When the surface is wetted by water drops, the refraction conditions at the surface are disturbed such that the light portion measured at the photo diode 4 varies as a function of the wetting condition.
The variation of the amplitude of the received light is evaluated for the purpose of controlling the windscreen wipers.
Such as system which offers the advantage of complete insensitivity towards foreign light is known from WO-A-95/01561.
In contrast to resistive, thermal or capacitive systems, optical rain sensors of the type described above were successful in the market. A certain restriction with regard to the mounting location is the necessity of direct optical contact with the windscreen inner surface and/or special measures to be taken for coupling the light beam in and out.
The employment of a video camera for detecting a wetting or soiling condition on the windscreen outer surface is known from DE-C-198 03 644, DE-A-197 49 331, DE-A-197 04 818, U.S. Pat. No. 6,097,024 and U.S. Pat. No. 4,867,561. However, these cameras of these systems exclusively serve for detecting rain drops.